The invention relates to a flexographic printing machine, especially for flexographic web printing, which comprises at least one printing unit, each with a printing cylinder or an endless printing belt drawn on round a back-up cylinder and a tension roller, with an impression cylinder, with an engraved roller transferring the ink to the printing cylinder or to the printing belt, and with an ink drier, and a paper web running through all the printing units and guided via an idler roller or idler rollers and a draw roller or draw rollers between the printing cylinder or printing belt and the impression cylinder and through the ink drier.
In conventional flexographic printing machines, a printing cylinder is used as a plate carrier or rubber cylinders with a structured surface are used. The disadvantage of these printing cylinders carrying the negative printing pattern on their surface is that they have to be exchanged for each printing order. U.S. Pat. No. 3,518,940 therefore proposes a printing mechanism comprising an endless belt composed of polyethylene terephthalate, on which the flexible printing plates are mounted.
Messrs. Conprinta Ltd., Zurich, further developed the flexographic printing machines with an endless printing belt and described these in their prospectus "Flexographic Printing Presses". The basic principle of these flexographic printing machines is illustrated in FIGS. 1 and 2 which represent prior art. A dimensionally stable endless belt 11 is drawn onto a back-up cylinder 12 and a tension roller 14. Flexible printing plates 16 are fastened to the outer face of this endless belt, thus forming a printing belt 10. The printing ink is transferred to the printing plates 16 from an engraved roller 18 which dips into an ink bath (not shown). Excess ink is scraped off by a doctor blade 20 designed in the form of a negative angle. The paper web 22 guided in the direction of the arrow is pressed onto the printing plates by an impression cylinder 24 and printed on one side.
An optionally mountable short endless belt 11' having only a few printing plates 16' and tensioned by a tension roller 14' is represented by broken lines.
FIG. 2 shows a cut-out from the printing belt 10. The endless belt 11 serving as a carrier consists of a polyester film, for example approximately 0.25 mm thick, especially polyethylene terephthalate. The physical properties of the transversely and longitudinally stretched film material are the same in all directions. The uniformity extends over a wide temperature and humidity range. Furthermore, the film-like belt material has good elongation and high impact resistance in transverse and longitudinal directions. Finally, the flexible film material of the endless belt is chemically resistant and withstands oils, greases, printing inks, etc. The longitudinal sides of the endless belt 11 are provided with a perforation 26, through which engage knobs of the spiked disks transporting the belt in a known way. The printing belt can thus be prevented from sliding on the rollers.
The flexible printing plates 16 conventionally consist of a photopolymer material or rubber and are glued to the endless belt 11 by means of a suitable adhesive. In the present example, the printing plates 16 have a structured surface 28 which produces the printing pattern.
The advantage of the above-described printing belt 10 is that the flexible printing plates 16 do not have to be mounted on a printing cylinder, but can be glued to the endless belt which is easily removed from the flexographic printing machine after use. All the printing belts used can be rolled onto a tube and stored in a space-saving way.
At present, flexographic prints of a width up to 2.5 m are produced by means of printing belts of a length of up to 4.5 m. The flexographic printing machines work at high speed and are equipped with checking instruments for the automatic monitoring and recording of the paper webs.
Flexographic printing machines constructed according to the Conprinta System are conventionally equipped with 3 to 10 printing units. Any printing unit not participating in the current printing process can be equipped for the next working step during that available idle time. During this, the printing belt no longer used is removed, the new printing belt is fitted and the printing ink is exchanged. This usually takes 5 to 15 minutes.
Of course, instead of the printing belt, individual printing units can also contain a conventional structured or non-structured printing cylinder. This applies primarily to smooth or structured lacquerings or to the application of a base color.
The inventor made it his object to improve further a flexographic printing machine of the type mentioned in the introduction and make its mode of operation more efficient. In particular, an engraved roller with the best possible screen should be available for each printing, without the need for assembly work which involves a high outlay and which is detrimental to the operating time of the machine.